The present invention relates to a method and an apparatus for the surface heat treatment of steel products by a laser beam. More particularly, the present invention is directed to such a method and an apparatus for such treatment of substantially V-shaped steel surfaces of steel products such as racks and gears.
It is known in various technologies that certain uneven surfaces of steel products, such as substantially V-shaped surfaces of steel products such as the teeth of racks and gears, must be treated, normally by a heat treatment, to increase the strength of such surfaces. Current practical industrial methods employed to achieve such strengthening include electromagnetic induction heating, carburizing, etc. However, such known strengthening methods are not altogether satisfactory from the viewpoint of strength, since the crests of such surfaces are generally excessively treated. Additionally, in carrying out such known processes, energy losses are substantial, and considerable distortion of the surfaces during the heat treatment results. Therefore, development work has been attempted to provide new methods for strengthening such surfaces.
Specifically, there have recently been conducted tests to employ electron beams and/or laser beams as sources of heat. Particularly, electron beam technology has already reached the level of practical application for welding. Heat treatment of flat steel surfaces by means of laser beam and/or electron beam technology will likely be achieved more and more in the future. However, the application of such technologies to uneven steel surfaces, such as those of racks and gears, still presents a number of practical problems, and as of yet no thoroughly practical industrial applications have been developed. More particularly, with reference now to FIG. 4 of the drawings, there will be illustrated the result of the heat treatment of a substantially V-shaped steel surface of a rack by means of a high energy beam in the form of a laser beam, such beam being applied to the steel surface in a heretofore known manner. More particularly, as will be apparent in FIG. 4, when the laser beam is projected substantially perpendicularly of the rack, and substantially perpendicularly of the bottom land of the V-shaped surface, the resultant hardened layer of the surface is much thicker at the bottom land of the surface than at the adjacent flank surfaces. This obviously results in an extremely uneven heat treatment, and the actual results of such heat treatment are highly undesirable. More particularly, in order to form hardened layers of desirable thickness on the flank surfaces, dissolution of the bottom land surface is inevitable. On the other hand, to form a hardened layer on the bottom land surface without dissolution thereof, it is virtually impossible to form hardened layers of suitable and sufficient thickness on the flank surfaces.
The above inherent disadvantages of the heat treatment shown in FIG. 4 particularly result when the laser beam has a Gaussian distribution of energy, wherein the energy is highest at the center of the beam and is increasingly lower toward the periphery of the beam. Consequently, the bottom land surface is heated to a much higher extent because the center of the beam with higher energy is projected onto the bottom land surface, while the flank surfaces are heated to a lesser extent by the periphery of the beam containing lower energy.
One previous attempt to solve the above discussed problem involves the use of a laser beam having an even energy distribution throughout the entire beam area, i.e. the use of a so-called "top-hat" type energy distribution. However, even when employing such a beam, it has still not been possible to obtain a desired uniformity of heat treatment of a surface profile as shown in FIG. 4, for the following reasons. That is, and again with reference to FIG. 4, if the angle formed between the flank surfaces is 40.degree., then the angle of incidence of the laser beam on the bottom land surface is 90.degree., that is the deviation from a perpendicular angle of incidence is 0.degree.. On the other hand, at a point on one of the flank surfaces, the deviation of the angle of incidence from perpendicular to the flank surface is 70.degree.. It will be apparent therefore that there will be a much higher degree of energy absorption at the bottom land surface than at the flank surfaces.
Moreover, those portions of the laser beam which are projected onto the flank surfaces are repeatedly reflected therefrom and directed toward the bottom land surface, thereby contributing to the further heating of the bottom land surface.
The results of the above phenomenon are that the hardened layer formed (shown by the hatched area in FIG. 4) is relatively thinner at the flank surfaces, i.e. whereat relatively thicker hardened layers are desired, and is thicker at the bottom land surface, i.e. whereat relatively thinner hardened layers are desired. These results have prevented the practical application of high energy beams, such as electron beams and laser beams, to the surface treatment of the surfaces of racks and gears.